1. Field of the Invention
This invention relates to a control circuit for an electrochemical cell coupled to a power grid. More specifically, it involves a control circuit having a first feedback loop with a slow time constant to form respective real power and reactive power error control signals for adjusting the power flow between the electrochemical cell and the power grid, and a second feedback loop with a fast time constant which controls the magnitude and phase of the voltage waveform on the AC side of the inverter connected between the electrochemical cell and the power grid.
2. Description of the Prior Art
Power producing and power storing devices are of particular interest to utility companies who operate extensive power grids which deliver large amounts of AC electrical power through a complex transmission network. Many different types of devices are known for producing electrical energy, one being the electrochemical cell which produces DC power by electrochemical means. Before the electrical power from such a DC device can be introduced into a power grid, it must be converted to AC electrical power. An inverter is one device capable of such a power conversion and a typical inverter includes a number of high-current capacity solid state switches, such as thyristors, which are periodically transitioned by a control circuit between the conductive and nonconductive states. In turn, this action alternatively connects the positive and negative inputs from the electrochemical cell through the inverter causing AC electrical energy to be presented to the power grid of the electrical utility.
A particular problem associated with the use of electrochemical cells of the battery type as a DC power source is that in the production of large amounts of electrical energy on a long term basis, many batteries consume the active chemical components, making up the cell. As a result, unless regenerated from an outside source, the power-producing capability of such electrochemical cells decline rapidly.
A type of electrochemical cell is known as the fuel cell and produces DC electrical energy directly from the oxidation of a fuel from an external source; therefore, it is capable of providing AC electrical power on a sustained basis. Of course, in the same manner as with the battery type of cell, the DC electrical energy must be converted to AC electrical energy for use by the utility. In addition to acting as a source of electrical energy, certain types of electrochemical cells are also suitable for use as a power storing device, this capability existing primarily where the electrochemical cell is regenerative. Thus, the regenerative cells have the ability to convert the active chemical elements, either internally stored or from an external source, into DC electrical energy and also have the capability of utilizing the DC energy to form active chemical fuels for later use. This bidirectional ability allows the electrochemical cell to be used as a power source during periods of peak power consumption, but during periods of lower power consumption when excess electrical energy is available, the cell converts the excess electrical energy into active chemical elements which is stored for later use.
Of interest is U.S. Pat. No. 3,991,319 issued Nov. 9, 1972 to J. Servos et al for STANDBY POWER SUPPLY SYSTEM. The patent describes a standby power supply including an inverter utilizing a direct current power supply. Synchronizing circuitry is provided to assure the phase and amplitude match of the inverter output to the AC line when the standby power source is utilized. The synchronizing circuit includes a phase detector which receives a feedback signal from the inverter and also senses the AC line voltage. The output from the phase detector is presented through a phase feedback switch to a voltage controlled oscillator to vary the frequency and phase of the output signal. A limitation of this particular system is that it is not suitable for controlling the direction and magnitude of both real power and reactive power between the source and the load.
Also of interest is the copending application U.S. application Ser. No. 059,725 filed July 23, 1979 by the same applicant, and assigned to the same assignee as the present invention, which describes a method for controlling the magnitude and direction of power flow through an inverter which is itself connected between an electrochemical cell and a power grid.